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Keep/discard elements based on their values

Source: R/keep.R
keep.Rd

keep() selects all elements where .p evaluates to TRUE; discard() selects all elements where .p evaluates to FALSE. compact() discards elements where .p evaluates to an empty vector.

Usage

keep(.x, .p, ..., .progress = FALSE)

discard(.x, .p, ..., .progress = FALSE)

compact(.x, .p = identity, .progress = FALSE)

Arguments

.x

A list or vector.

.p

A predicate function (i.e. a function that returns either TRUE or FALSE) specified in one of the following ways:

  • A named function, e.g. is.character.

  • An anonymous function, e.g. \(x) all(x < 0) or function(x) all(x < 0).

Wrap a function with in_parallel() to declare that it should be performed in parallel. See in_parallel() for more details. Use of ... is not permitted in this context.

...

Additional arguments passed on to .p.

.progress

Whether to show a progress bar. Use TRUE to turn on a basic progress bar, use a string to give it a name, or see progress_bars for more details.

Details

In other languages, keep() and discard() are often called select()/ filter() and reject()/ drop(), but those names are already taken in R. keep() is similar to Filter(), but the argument order is more convenient, and the evaluation of the predicate function .p is stricter.

.p is called once for each element of .x, as .p(.x[[1]], ...), .p(.x[[2]], ...), ..., and must return a single TRUE or FALSE.

See also

keep_at()/discard_at() to keep/discard elements by name.

Examples

rep(10, 10) |>
  map(sample, 5) |>
  keep(function(x) mean(x) > 6)
#> [[1]]
#> [1]  5  2  8  6 10
#> 
#> [[2]]
#> [1] 4 9 8 5 6
#> 
#> [[3]]
#> [1]  7 10  8  3  4
#> 

# Or use shorthand form
rep(10, 10) |>
  map(sample, 5) |>
  keep(\(x) mean(x) > 6)
#> [[1]]
#> [1] 10  8  9  5  1
#> 
#> [[2]]
#> [1]  2 10  7  5  8
#> 

# Using a string instead of a function will select all list elements
# where that subelement is TRUE
x <- map(1:5, \(.x) list(a = runif(1) > 0.5, b = sample(10)))
x
#> [[1]]
#> [[1]]$a
#> [1] FALSE
#> 
#> [[1]]$b
#>  [1]  1  7  4  5  3  9  2 10  8  6
#> 
#> 
#> [[2]]
#> [[2]]$a
#> [1] FALSE
#> 
#> [[2]]$b
#>  [1]  4  8  9 10  7  3  2  5  1  6
#> 
#> 
#> [[3]]
#> [[3]]$a
#> [1] FALSE
#> 
#> [[3]]$b
#>  [1]  3  9  6 10  2  5  8  4  7  1
#> 
#> 
#> [[4]]
#> [[4]]$a
#> [1] FALSE
#> 
#> [[4]]$b
#>  [1]  2 10  1  5  6  9  4  7  3  8
#> 
#> 
#> [[5]]
#> [[5]]$a
#> [1] FALSE
#> 
#> [[5]]$b
#>  [1]  7  9  6  4  8  2  1  5 10  3
#> 
#> 
x |> keep("a")
#> list()
x |> discard("a")
#> [[1]]
#> [[1]]$a
#> [1] FALSE
#> 
#> [[1]]$b
#>  [1]  1  7  4  5  3  9  2 10  8  6
#> 
#> 
#> [[2]]
#> [[2]]$a
#> [1] FALSE
#> 
#> [[2]]$b
#>  [1]  4  8  9 10  7  3  2  5  1  6
#> 
#> 
#> [[3]]
#> [[3]]$a
#> [1] FALSE
#> 
#> [[3]]$b
#>  [1]  3  9  6 10  2  5  8  4  7  1
#> 
#> 
#> [[4]]
#> [[4]]$a
#> [1] FALSE
#> 
#> [[4]]$b
#>  [1]  2 10  1  5  6  9  4  7  3  8
#> 
#> 
#> [[5]]
#> [[5]]$a
#> [1] FALSE
#> 
#> [[5]]$b
#>  [1]  7  9  6  4  8  2  1  5 10  3
#> 
#> 

# compact() discards elements that are NULL or that have length zero
list(a = "a", b = NULL, c = integer(0), d = NA, e = list()) |>
  compact()
#> $a
#> [1] "a"
#> 
#> $d
#> [1] NA
#>